Spherical transducer



A. I. DRANETZ ET AL SPHERICAL TRANSDUCER June 7, 1960 Filed Dec. 5. 1954INVENTORS ABRAHAM I. DRANETZ HUGH J CULLIN 2 QTTOPNEY numeral 13designates the inner electrode.

SPHERICAL TRANSDUCER Abraham I. Dranetz and Hugh J. Cullin, Metuchen,N.J., assignors to Gulton Industries, Inc, a corporation of New JerseyFiled Dec. 3, 1954, Ser. No. 472,966

2 Claims. ((1310-85) A still further object of our invention is toprovide a transducer which may be employed in conjunction with adirectional reflector or other directive device.

Other objects and advantagesof our invention will be apparent during thecourse of the following descrip tron.

In the accompanying drawings, forming a part of this application, and inwhich like numerals are employed to designate like parts throughout thesame.

Figure 1 is a vertical view of an embodiment of our invention,

Figure 2 is a vertical section of the embodiment of Figure l, and

Figure 3 is employed to illustrate the theory underlying our invention.I

In the drawings, wherein for the purpose of illustration, is shown apreferred embodiment of our invention, the numeral designates themicrophone assembly, the numeral 11 designates the electromechanicaltransducer; the numeral 12 designates the outer electrode and the Thenumeral 14 designates the center conductor which is electricallyconnected to the inner electrode 13 and the numeral 15 designates theelectrical connection from the cable shield 16 to the outer electrode12.

The numeral 17 designates the compliant insulator and the numeral 18designates the insulating outer jacket of the cable 20. The numeral 19designates the sealing compound and the numeral 21 designates the cableclamping surface and the numeral 22 designates the hole in the spherethrough which the connections are made.

Microphone assembly 10 is spherical so that its sur face looks the sameto sound which is impinged on it from any direction. In the embodimentof our invention shown in the figures, the active transducer element-1-1 is a hollow spherical ceramic to whose surfaces have been appliedelectrodes 12 and 13. A small hole 22 is cut in the sphere so that cablemay be inserted and the necessary electrical connections made fromcenter condoctor 14 to inner electrode 13 and from the shield 16 toouter electrode 12. The insulating outer jacket of cable 20 is cut backwhere cable 20 enters hole 22 and a plastic casting compound 19 isinserted in the hole 22 alongside cable 20 and allowed to flow inside 10and to harden and set. This insures good mechanical bonding of the cable20 to the microphone assembly 10. Compliant insulator 17 is bonded to.10 so as to completely surround hole 22 and is compliant so that the2,939,970 Patented June 7, 1960 2 transducer takes all the acousticforce which is applied to the microphone.

Other embodiments and methods for mounting and assembling transducers,produced in accordance with our invention, may be employed. One suchmethod contemplates inserting a rubber or plastic-covered flexibleshielded cable in hole 22 in the-transducer 11 and sealing hole 22 withhard glass or a like material.

The electrodes may be on both surfaces as shown in the figures or may beapplied to either surface depending on the particular application. Or, astrain gauge wire on a metallic sphere may be employed'as the activeele- 'ment in the device. I

When microphone assembly 10 is placed in an acoustic field, the soundimpinges on 10 radially and compresses and expands transducer 11 inradial mode. sonant frequency of 11 in the radial mode is very highwhich fact, coupled with the low mechanical Q of the sphere, permits themicrophone to possess a broad frequency response beyond that attainablewith diaphragm type microphones. .We have found that themicrophones andhydrophones produced in accordance with our-invention haveomnidirectional response in both the yerfi cal and horizontaldirections. For the purpose of this discussion the vertical planev isthat which contains the axis of cable 20 and the horizontal plane is theplane which is perpendicular to the axis of cable 20.

Units produced accordingto our invention are particularly useful in highintensityfields and may be used as microphones, hydrophones, projectorsor loudspeakers. This is due to the fact that these spherical sensitiveelements are electro-acoustically reversible.

In the field of subaqueous sound transmisison and v reception,hydrophones have been most generally produced in the shape of hollowcylinders. The resonant frequency of a sphere in radial mode isapproximately 1.7 times higher than that of a cylinder of thesame'radius and in addition, there is no length mode of vibration toconsider when spheres are used instead of cylinders.

In addition, we have: m=R dy R dx t 'p; i

'd R d0 where p=density t=thickness 0=time R =radius From which:

F, ma R dxdytp Likewise, the fundamental equation for the elastic forceis given by:

= i E! 7 R0 The re- 3 where 2737i: i' i .i.s=str,ess f i E= Youngsmodulus =Poissons ratio The force in the radial direction is given by:

F ,=F sin dy-l-F sin dx and for small values of dy and dx,

assuming.dy=dx, we obtain: Fr=2Fdy. I The force F is equal to the stresstimes the area or F-=S t Rode; w 3am from which, the radial force, F,,is given by;

' F,.=2E'ARtdxdy 1; 'For equilibrium, the force of inertia is equal tominus the elastic force, so that:

' AR=R-R =A (amplitude of motion) and since f, (frequency of resonance)=5 The mechanical figure of merit, Qifor a subaqueous sound receiver(hydrophone) is given by:

cycles/see.

where LB n...

c =velocity of sound in the ceramic p =density of the ceramict=thickness of the ceramic wall i a=mean radius c =velocity of sound inwater p =density of the water '=Poissons ratio (.32 for barium titanate)For barium titanate and sea water, we obtain:

and if t is small with respect to a Q is low.

When devices made in accordance with our invention are employed forsubaqueous sound reception or transmission, they are suitably treated sothat the liquid in which they are'immersed cannot enter into contactwith 40 the active element or the electrodes.

Our invention contemplates-the use of the units, produced in accordancetherewith, in directional microphones, hydrophones, projectors andloudspeakers. The location of the unit at the focus of a parabolicreflector, for instance, results in a very good directionalelectroacoustical device. This is due to the fact that the inherentlyuniform response of the unit in free space permits sound pickup from anydirection and all sound reflected from the parabolic reflector onto thesensitive element is unattenuated due to the direction of incidence tothe sensitive element. The unit is equally well adapted to use inconjunction with any other type of directional device. While we havedescribed our invention by means of specific examples and in a specificembodiment, we do not wish to be limited thereto for obviousmodifications will occur to those skilled in the art without departing"from the'spiri't and scope of the invention.

' Having thus described our invention, we claim: 1. ,Anelectroacoustical transducer for insertion in an acoustic transmissionmedium comprising a hollow spherical, piezoelectric ceramic element insaid medium, said ceramic element having an opening therein, electrodesaflixed to the inner and outer surfaces of said-ceramic element, a cablecomprising two electrical conductors and an outer compliant sheath, saidcable being aflixed to said ceramic element at said opening, means forconnecting one of said electrical conductors to the inner of saidelectrodes and the other of said electrical conductors to the outer'ofsaid electrodes, a compliant member surrounding said cable in contactwith said ceramic element, and meansrwithin said opening for sealingsaid 1 opening whereby the interior of said ceramicelement is isolatedfrom said medium; a 76 Z, The invention as described in claim 1 whereinthe 5 ratio of the thickness of said ceramic element to the radius ofsaid ceramic element is small.

References Cited in the file of this patent UNITED STATES PATENTS1,919,796 Marrison July 25, 1933 2,399,082 Wainer Apr. 23, 19422,420,864 Chilowsky May 20, 1947 2,556,558 Silverman June 12, 1951Williams Aug. 21, 1951 Langevin Oct. 16, 1956 Camp- June '11, 1957FOREIGN PATENTS Great Britain July 2, 1910 Great Bntain Aug. 21, 1947OTHER REFERENCES Piezotronics, Brush Electronics 00., July 1953, p. 3.

